Does this work better if you change the font, and if yes, which commonly available font works best?

If not, does the post look properly on the front-end and if you load it first in the Text editor (switch to Text and reload the page)?

Thank you, @azaozz,

I think the problem is "Georgia" font and the good choice is as same as left menu's font family, or the set: "Segoe UI",Roboto, sans-serif. I tried with "Segoe UI" (default windows 8 - 10 font) and the Vietnamese character display well.

(following on from Slack discussion) I'm not a designer or front-end guy but I'm concerned that the proposed patch is a big change for a minor release. Going from a sans-serif stack to a serif stack seems like one which could be jarring and unexpected.

Do we have a native system font serif stack? Could that be a viable alternative?

The operating system’s UI font is used for any text that’s part of the WordPress user interface. In other contexts, like the Editor, we continue to use a serif system typeface, Georgia. This creates a clear typographic distinction between text that is part of the interface, and text that is part of the user’s content.

No, not a regression in 4.6 but the fix is so trivial and the impact is so large that it will be pity not fixing it now. Also think we should do #37808 which is somewhat similar, although that would mean backporting the fix and pushing an update to TinyMCE.

...would any other languages profit from such a fix as well?

Unfortunately we wouldn't know unless the users or developers that understand these languages speak up.